Gas mixture for electric flashtubes



United States Patent GAS MIXTURE FOR ELECTRIC FLASHTUBES John L. Turner, Needham, Mass., assignor to EG & G, Inc., Bedford, Mass., a corporation of Massachusetts No Drawing. Continuation-impart of application Ser. No. 262,364, Mar. 4, 1963. This application May 18, 1966,

Ser. No. 551,172

1 Claim. (Cl. 252-372) ABSTRACT OF THE DISCLOSURE Gas mixture for electric flashtubes consisting of 90% to 97% xenon, 2% to 9% hydrogen and 1% to argon.

This application is a continuation-an-part of application Ser. No. 262,364 filed Mar. 4, 1963, now abondoned. This invention relates generally to gaseous-dischargedevices and more particularly to the gas mixture employed therein.

Gaseous discharge devices having an inert atmosphere for the ionizing medium are well known. A problem that has long existed in such devices is hold-over. Hold-over is the failure of the device to extinguish completely immediately after a discharge. It is due primarily to relatively slow deionization times of the gaseous medium and a residual current flowing therethrough. It is most serious when devices such as electric flashtubes are operated at high repetition rates in a stroboscope or in flash photography where very short duration light flashes are desired.

The fact that hydrogen mitigates deionization problems of inert atmospheres such as neon, xenon, helium, krypton, nitrogen, and radon is well known. The addition of a small amount of hydrogen to the inert atmosphere shortens the deionization time of a gaseous discharge device; hence, duration of the residual current is reduced. In stroboscopy and high-speed photography, this result is very desirable since the duration of the flash is greatly reduced. However, the addition of hydrogen to the inert atmosphere produces its own bad side effects such as reduced light output and a higher starting voltage. In one test of a triggered gaseous discharge device a constantamplitude pulse voltage is applied to the trigger electrode and the voltage applied across the main electrodes of the device is gradually increased until an arc discharge occurs across the main electrodes. The voltage at which the arc discharge occurs is defined as the starting voltage. This voltage should be low to permit the device to operate from a low voltage supply. Obviously if this voltage is too great the power supply necessary to operate the device would be expensive and cumbersome.

Thus, a designer is faced with the dilemma of choosing between a gaseous discharge device that has a low starting voltage, long deionization time, and, in the case of a flashtube, high light output; or a device that has a higher starting voltage, short deionization time and a lower light output.

It is, therefore, an object of my invention to mitigate the bad side effects of hydrogen in gaseous-discharge devices. Other objects will become apparent by reference to the following description.

Briefly, my invention resides in the wholly unexpected discovery that the addition of argon to a mixture of xenon and hydrogen mitigates the bad side effects of hydrogen. It is well known that the addition of argon to neon in glow discharge tubes reduces the starting voltages of such tubes. Likewise, the addition of argon to a mixture of neon and a small quantity of hydrogen reduces the starting voltage. The reason for this is that the metastable energy of neon is sufficient to ionize argon. The results are that the ionization coeflicients of neon-argon mixtures are much higher and that the Paschen curve minimums are much lower than for pure neon or any other of the inert gases. The metastable levels of nitrogen, krypton, hydrogen, xenon, and mercury vapor are lower than that of argon. Hence, those skilled in the art would not expect such gases to ionize argon with the same result as with neon. My experiments indicate that this is so with respect to xenon; that is, the addition of a small amount of argon to pure xenon gas does not reduce its starting voltage.

One day while experimenting with Xenon-hydrogen gas mixtures, I made a small number of flashtubes in which the starting voltage was considerably lower than the starting voltage normally obtained for this mixture. I tried to repeat the experiment but without success. The addition of hydrogen continued to cause the known increase in starting voltage above the starting voltage for pure xenon. While searching for an explanation I discovered that possibly a leak in the argon line of my apparatus had contaminated the earlier gas mixture with argon. I then tried a mixture of xenon, hydrogen and argon and found that the starting voltage was considerably below the starting voltage of the xenon and hydrogen mixture.

A typical gaseous discharge device that is used for highspeed photography and stroboscopy is the flashtube disclosed in US. Letters Patent No. 2,977,508 issued Mar. 28, 1961 to Kenneth J. Germeshausen, and assigned to the same assignee as the present invention. It is to be understood that I refer to this flashtube for purposes of discussion and do not intend to be limited thereto.

When a 0.27 microfarad capacitor charged to 640 volts was discharged through such a flashtube that had been filled with xenon to a pressure of about 400 mm. Hg absolute, the light output was approximately 19,500 horizontal peak candle power. However, the flashtube exhibited hold-over characteristics which limited its use for certain applications. By adding about 8 mm. Hg absolute of hydrogen, the hold-over characteristic was reduced drastically, but the light output dropped to 17,500 horizontal peak candle power. Also the starting voltage rose from a value of 300 volts for pure xenon to 400 volts for the xenon-hydrogen mixture.

I added about 4 mm. Hg absolute or argon. The light output increased from 17,500 to 18,750 horizontal peak candle power. The starting voltage dropped from 400 volts to 350 volts. There was no increase in the hold-over characteristics. To summarize, the argon had no effect on the beneficial characteristics of hydrogen, while alleviating hydrogens bad side effects.

By making the proportion of hydrogen equal (4 mm. Hg absolute) to that of the argon (4 mm. Hg absolute) in another gaseous mixture the same beneficial results were obtained initially, but as the flashtube aged, the beneficial effects of hydrogen deteriorated. It is believed that this deterioration is caused by gettering of the hydrogen or its adsorption by the flashtubes components. Because of the similarity between the results of the first mixture and the initial results of the second mixture, it is believed that a 1:1 ratio of hydrogen to argon is desirable during the life of the flashtube. However, the usual variations in percentages of gases are tolerable because it is not feasible to hold to an exact 1:1 ratio. Thus, if none of the hydrogen will be adsorbed, it will not be necessary to vary from the 1:1 ratio of hydrogen to argon when the flashtube is being filled. However, if hydrogen will be adsorbed, the initial ratio should be changed proportionally. Because of the gettering or adsorption of hydrogen within a gaseousdischarge device, the percentage range of gases employed as the fill is about: xenon %-97%, hydrogen 2%9%, and argon 1%-5%.

Twenty of the aforementioned flashtubes were constructed and divided into four groups of five tubes each.

3 Each group was filled with different gases and their characteristics were determined. The fill gases were as follows:

Table I presents the characteristics found for the various fill gases. Note that peak light drops considerably from Group A to Group B, but rises substantially for Group D. The starting voltage increases from Group A to Group B and then drops back half way for Group D. The hold-over of Group A is eliminated completely in Group B and in Group D. There was no improvement in hold-over in Group C compared with Group A while there was an adverse effect upon peak light output. More important though, is the fact that there was no change in starting voltage from Group A to Group C.

This demonstrates experimentally that the metastable levels of Xenon will not ionize argon and that the addition of argon to Xenon will not reduce the starting voltage,

It will be apparent to those skilled in the art that my discovery can be used in any gaseous-discharge device wherein the discharge is in the form of an arc discharge, as in' a flashtube. Thus, the glass envelope of a flashtube can be replaced by an envelope of opaque material.

Further modifications may be made by those skilled in the art without departing from the spirit and scope of my invention as set forth in the appended claim.

I claim: 1. A gaseous medium for an electric flashtube consisting of:

90% to 97% xenon 2% to 9% hydrogen 1% to 5% argon References Cited UNITED STATES PATENTS 1,481,422 1/ 1924. Holst et al. 1,918,309 7/ 1933 Weinhart. 1,949,617 3/ 1934 Michelssen.

1 2,038,825 .4/1936 Canady.

2,847,614 8/1958 Anderson et al.

LEON ZrrvER, Primary Examiner.

H; MARS, Assistant Examiner. 

